Machines, including skidsteer loaders, track-type tractors, hydraulic tracked excavators, military tanks, and other similar types of heavy construction and mining equipment are used for a variety of tasks. These machines typically include ratio control devices that transmit torque from an engine to one or more traction devices that move the machine. These ratio control devices generally include a hydrostatic transmission having a common pump and two motors independently driven by pressurized fluid from the pump, or two completely separate pump and motor combinations.
Two independent motors are required for steering of the machine. That is, during a steering event, one or more driven traction devices located on one side of the machine may be caused to rotate at a slower speed than or in a direction opposite to one or more driven traction devices located on an opposing side of the machine to cause the machine to turn. Without independent driving rotations of the traction devices, these types of machine would not be steerable.
Temperature differentials (or hot spots) inherently occur in hydrostatic transmissions because of differences in fluid pressure across the transmission. Flushing the entire charge flow of a closed hydrostatic loop can reduce these hot spots. With full charge flushing, the entire flow of a closed hydrostatic loop discharges into a storage tank. Simultaneously, the system pumps in cooler replacement fluid. In larger vehicles, providing multiple charge relief valves (i.e., one or more relief valves for each pump/motor pairing) presents a cost-effective way to attain full charge flushing of a closed hydrostatic loop.
Although perhaps effective for large vehicles, in smaller vehicles, such as a skidsteer loader, providing multiple charge relief valves can be cost prohibitive. One low cost alternative for flushing a closed hydrostatic loop is disclosed in U.S. Pat. No. 4,332,134 (the '134 patent) issued to Cochran et al. on Jun. 1, 1982. The '134 patent discloses a bleed valve for a closed loop hydrostatic transmission system having a charging line, a low pressure loop line, a high pressure loop line, and a low pressure drain line. In order to keep the hydrostatic power transmission loop lines cool, a controlled bleed can be implemented. During the controlled bleed, fluid discharges through the bleed valve at a low rate to a tank or other low pressure drain as the lost oil is simultaneously replaced by cool charging oil. The bleed valve includes a pressure sensing port, a poppet means, and a means for balancing pressure-forces applied to the poppet means. When charging pressure in the closed hydrostatic loop becomes low, the balancing means and poppet means cooperate to shut off the bleed valve. Because the system of the '134 patent can provide flushing with a single valve, it may be a low cost alternative to providing multiple valves.
Although the system of the '134 patent may provide a low cost solution to cooling a hydrostatic transmission, it may provide insufficient cooling. That is, the cooling flow may be interrupted when the pressure of the circuit is too low (i.e. the system does not provide full flow flushing). This interruption may result in periods of undesired high temperatures.
The flushing system of the present disclosure solves one or more of the problems set forth above.